This research examines the degree to which different phonological processes in speech production interact with one another. Theories of speech production typically assume two distinct stages of phonological processing. First, lexical phonological processes retrieve coarse-grained representations of word form from long term memory. Phonetic processes then generate more fine-grained aspects of phonological structures. Although theories agree that two such processing stages are present, they disagree on the degree to which they interact with one another. The experiments proposed here test one specific proposal for the interaction of these stages: the phonological cascade hypothesis. This hypothesis proposes that although interaction between processes is present, it is restricted. Cascading activation supports interaction by allowing effects that originate within lexical phonological processing (e.g., word-level variables such as word frequency) to influence phonetic processing. Interaction is crucially limited by blocking feedback from phonetic to lexical phonological processes. This restriction on interaction prevents effects originating within phonetic processing (e.g., effects of fine-grained structure such as articulatory similarity) from directly influencing lexical phonological processing. Two series of experiments test this hypothesis. The first examines the performance of individuals with acquired deficits to lexical phonological or phonetic processing. The second examines experimentally induced speech errors in neurologically intact participants. The phonological cascade hypothesis predicts that speech errors generated within lexical phonological processes (either experimentally or via neurological damage) should show sensitivity to word-level variables (e.g., word frequency) but, due to the lack of feedback, be uninfluenced by fine-grained aspects of phonological structure (e.g., articulatory similarity). In contrast, due to cascade, speech errors generated during phonetic processing should be sensitive to both types of variables. Project Narrative: By addressing a relatively unexplored issue in how the sound structure of words is processed to support the production of speech, the experiments proposed here will provide the basis for richer, more detailed theories of language production. Furthermore, increasing our understanding of how the adult language processing system breaks down in aphasia may assist the development of more refined therapies for acquired speech production impairments.